1. Field of the Invention
The present invention relates to the field of non-aqueous, active metal electrochemical cells and, more particularly, to an improved cathode for high energy density cells which allows the cells to safely withstand extensive thermal incursions associated with incendiary or short circuit conditions.
2. Background Art
Much work has been done in the field of high energy battery systems utilizing highly reactive anode materials such as alkali metals in combination with non-aqueous electrolytes. In these cells, the preferred alkali metal anode is lithium. The electrolyte normally includes a solute which is commonly a metal salt or complex metal salt of the anode metal dissolved in a compatible non-aqueous solvent depolarizer. Examples of such salts include lithium tetrachloroaluminate and the solvent depolarizers typically include those containing sulfur dioxide (SO.sub.2) and oxyhalides including, thionyl chloride (SOCl.sub.2) or sulfuryl chloride (SO.sub.2 Cl.sub.2). A relatively inert cathode collector such as a nickel or stainless steel grid covered with a porous layer of compressed carbon black such as an acetylene black, completes the cell couple.
Safety is one of the major concerns associated with high energy density systems including lithium-thionyl chloride (Li/SOCl.sub.2) couples. The possibility of such a cell or battery rupturing or exploding under a variety of abuse conditions has long been a drawback to certain applications. The problem appears to be centered around thermal incursions that may occur under short circuit or incendiary conditions. Generally, when the temperature in a cell is sufficiently elevated to occasion the melting of the anode, the molten anode material then reacts with other chemical species in the cell, often rather violently. Increased internal pressure during excessive heat generation is another safety hazard.
Ceramic materials in contact with the anode have been used as neutralizing agents to react with it to reduce the hazardous effect of the liquified anode material at the anode melting temperature before more violent reactions take place with other cell species. This approach is described by Catanzarite in U.S. Pat. No. 4,407,910. A ceramic separator has been proposed in U.S. Pat. No. 4,598,029 by N. Doddapaneni and D.L. Chua, co-inventors in the present application. That reference deals with an approach to minimizing hazards in which a porous ceramic separator is used to prevent excessive voltage drop during cell reversal under forced overdischarge.
Despite these prior attempts to solve the safety problems of these cells associated with the propagation of thermal run away reactions, none have solved the problem with respect to the occurrence of direct internal shorts. Shorts may be due to defective construction or caused by violent and external trauma such as nail piercing or bullet penetration associated with military applications of the cells.